A Short History of Nearly Everything- 第43部分

today；　the　site　of　voorhies’s　discovery　is　called　ashfall　fossil　beds　state　park；　and　it　has　astylish　new　visitors’　center　and　museum；　with　thoughtful　displays　on　the　geology　of　nebraskaand　the　history　of　the　fossil　beds。　the　center　incorporates　a　lab　with　a　glass　wall　throughwhich　visitors　can　watch　paleontologists　cleaning　bones。　working　alone　in　the　lab　on　themorning　i　passed　through　was　a　cheerfully　grizzled…looking　fellow　in　a　blue　work　shirt　whomi　recognized　as　mike　voorhies　from　a　bbc　television　documentary　in　which　he　featured。

they　don’t　get　a　huge　number　of　visitors　to　ashfall　fossil　beds　state　park—it’s　slightly　inthe　middle　of　nowhere—and　voorhies　seemed　pleased　to　show　me　around。　he　took　me　to　thespot　atop　a　twenty…foot　ravine　where　he　had　made　his　find。

“it　was　a　dumb　place　to　look　for　bones；”　he　said　happily。　“but　i　wasn’t　looking　for　bones。　iwas　thinking　of　making　a　geological　map　of　eastern　nebraska　at　the　time；　and　really　just　kindof　poking　around。　if　i　hadn’t　gone　up　this　ravine　or　the　rains　hadn’t　just　washed　out　that　skull；i’d　have　walked　on　by　and　this　would　never　have　been　found。”　he　indicated　a　roofedenclosure　nearby；　which　had　bee　the　main　excavation　site。　some　two　hundred　animalshad　been　found　lying　together　in　a　jumble。

i　asked　him　in　what　way　it　was　a　dumb　place　to　hunt　for　bones。　“well；　if　you’re　looking　forbones；　you　really　need　exposed　rock。　that’s　why　most　paleontology　is　done　in　hot；　dry　places。

it’s　not　that　there　are　more　bones　there。　it’s　just　that　you　have　some　chance　of　spotting　them。

in　a　setting　like　this”—he　made　a　sweeping　gesture　across　the　vast　and　unvarying　prairie—“you　wouldn’t　know　where　to　begin。　there　could　be　really　magnificent　stuff　out　there；　butthere’s　no　surface　clues　to　show　you　where　to　start　looking。”

at　first　they　thought　the　animals　were　buried　alive；　and　voorhies　stated　as　much　in　anational　geographic　article　in　1981。　“the　article　called　the　site　a　‘pompeii　of　prehistoricanimals；’　”　he　told　me；　“which　was　unfortunate　because　just　afterward　we　realized　that　theanimals　hadn’t　died　suddenly　at　all。　they　were　all　suffering　from　something　calledhypertrophic　pulmonary　osteodystrophy；　which　is　what　you　would　get　if　you　were　breathing　alot　of　abrasive　ash—and　they　must　have　been　breathing　a　lot　of　it　because　the　ash　was　feetthick　for　hundreds　of　miles。”　he　picked　up　a　chunk　of　grayish；　claylike　dirt　and　crumbled　itinto　my　hand。　it　was　powdery　but　slightly　gritty。　“nasty　stuff　to　have　to　breathe；”　he　went　on；“because　it’s　very　fine　but　also　quite　sharp。　so　anyway　they　came　here　to　this　watering　hole；presumably　seeking　relief；　and　died　in　some　misery。　the　ash　would　have　ruined　everything。　itwould　have　buried　all　the　grass　and　coated　every　leaf　and　turned　the　water　into　an　undrinkablegray　sludge。　it　couldn’t　have　been　very　agreeable　at　all。”

the　bbc　documentary　had　suggested　that　the　existence　of　so　much　ash　in　nebraska　was　asurprise。　in　fact；　nebraska’s　huge　ash　deposits　had　been　known　about　for　a　long　time。　foralmost　a　century　they　had　been　mined　to　make　household　cleaning　powders　like　et　andajax。　but　curiously　no　one　had　ever　thought　to　wonder　where　all　the　ash　came　from。

“i’m　a　little　embarrassed　to　tell　you；”　voorhies　said；　smiling　briefly；　“that　the　first　i　thoughtabout　it　was　when　an　editor　at　the　national　geographic　asked　me　the　source　of　all　the　ash　andi　had　to　confess　that　i　didn’t　know。　nobody　knew。”

voorhies　sent　samples　to　colleagues　all　over　the　western　united　states　asking　if　there　wasanything　about　it　that　they　recognized。　several　months　later　a　geologist　named　billbonnichsen　from　the　idaho　geological　survey　got　in　touch　and　told　him　that　the　ash　matcheda　volcanic　deposit　from　a　place　called　bruneau…jarbidge　in　southwest　idaho。　the　event　thatkilled　the　plains　animals　of　nebraska　was　a　volcanic　explosion　on　a　scale　previouslyunimagined—but　big　enough　to　leave　an　ash　layer　ten　feet　deep　almost　a　thousand　miles　awayin　eastern　nebraska。　it　turned　out　that　under　the　western　united　states　there　was　a　hugecauldron　of　magma；　a　colossal　volcanic　hot　spot；　which　erupted　cataclysmically　every600；000　years　or　so。　the　last　such　eruption　was　just　over　600；000　years　ago。　the　hot　spot　isstill　there。　these　days　we　call　it　yellowstone　national　park。

we　know　amazingly　little　about　what　happens　beneath　our　feet。　it　is　fairly　remarkable　tothink　that　ford　has　been　building　cars　and　baseball　has　been　playing　world　series　for　longerthan　we　have　known　that　the　earth　has　a　core。　and　of　course　the　idea　that　the　continents　moveabout　on　the　surface　like　lily　pads　has　been　mon　wisdom　for　much　less　than　a　generation。

“strange　as　it　may　seem；”　wrote　richard　feynman；　“we　understand　the　distribution　of　matterin　the　interior　of　the　sun　far　better　than　we　understand　the　interior　of　the　earth。”

the　distance　from　the　surface　of　earth　to　the　center　is　3；959　miles；　which　isn’t　so　very　far。

it　has　been　calculated　that　if　you　sunk　a　well　to　the　center　and　dropped　a　brick　into　it；　it　wouldtake　only　forty…five　minutes　for　it　to　hit　the　bottom　（though　at　that　point　it　would　beweightless　since　all　the　earth’s　gravity　would　be　above　and　around　it　rather　than　beneath　it）。

our　own　attempts　to　penetrate　toward　the　middle　have　been　modest　indeed。　one　or　two　southafrican　gold　mines　reach　to　a　depth　of　two　miles；　but　most　mines　on　earth　go　no　more　thanabout　a　quarter　of　a　mile　beneath　the　surface。　if　the　planet　were　an　apple；　we　wouldn’t　yethave　broken　through　the　skin。　indeed；　we　haven’t　even　e　close。

until　slightly　under　a　century　ago；　what　the　best…informed　scientific　minds　knew　aboutearth’s　interior　was　not　much　more　than　what　a　coal　miner　knew—namely；　that　you　could　digdown　through　soil　for　a　distance　and　then　you’d　hit　rock　and　that　was　about　it。　then　in　1906；an　irish　geologist　named　r。　d。　oldham；　while　examining　some　seismograph　readings　from　anearthquake　in　guatemala；　noticed　that　certain　shock　waves　had　penetrated　to　a　point　deepwithin　the　earth　and　then　bounced　off　at　an　angle；　as　if　they　had　encountered　some　kind　ofbarrier。　from　this　he　deduced　that　the　earth　has　a　core。　three　years　later　a　croatianseismologist　named　andrija　mohorovi？i′c　was　studying　graphs　from　an　earthquake　in　zagrebwhen　he　noticed　a　similar　odd　deflection；　but　at　a　shallower　level。　he　had　discovered　theboundary　between　the　crust　and　the　layer　immediately　below；　the　mantle；　this　zone　has　beenknown　ever　since　as　the　mohorovi？i′c　discontinuity；　or　moho　for　short。

we　were　beginning　to　get　a　vague　idea　of　the　earth’s　layered　interior—though　it　really　wasonly　vague。　not　until　1936　did　a　danish　scientist　named　inge　lehmann；　studyingseismographs　of　earthquakes　in　new　zealand；　discover　that　there　were　two　cores—an　innerone　that　we　now　believe　to　be　solid　and　an　outer　one　（the　one　that　oldham　had　detected）　thatis　thought　to　be　liquid　and　the　seat　of　magnetism。

at　just　about　the　time　that　lehmann　was　refining　our　basic　understanding　of　the　earth’sinterior　by　studying　the　seismic　waves　of　earthquakes；　two　geologists　at　caltech　in　californiawere　devising　a　way　to　make　parisons　between　one　earthquake　and　the　next。　they　werecharles　richter　and　beno　gutenberg；　though　for　reasons　that　have　nothing　to　do　with　fairnessthe　scale　became　known　almost　at　once　as　richter’s　alone。　（it　has　nothing　to　do　with　richtereither。　a　modest　fellow；　he　never　referred　to　the　scale　by　his　own　name；　but　always　called　it“the　magnitude　scale。”）the　richter　scale　has　always　been　widely　misunderstood　by　nonscientists；　though　perhapsa　little　less　so　now　than　in　its　early　days　when　visitors　to　richter’s　office　often　asked　to　seehis　celebrated　scale；　thinking　it　was　some　kind　of　machine。　the　scale　is　of　course　more　anidea　than　an　object；　an　arbitrary　measure　of　the　earth’s　tremblings　based　on　surfacemeasurements。　it　rises　exponentially；　so　that　a　7。3　quake　is　fifty　times　more　powerful　than　a6。3　earthquake　and　2；500　times　more　powerful　than　a　5。3　earthquake。

at　least　theoretically；　there　is　no　upper　limit　for　an　earthquake—nor；　e　to　that；　a　lowerlimit。　the　scale　is　a　simple　measure　of　force；　but　says　nothing　about　damage。　a　magnitude　7quake　happening　deep　in　the　mantle—say；　four　hundred　miles　down—might　cause　no　surfacedamage　at　all；　while　a　significantly　smaller　one　happening　just　four　miles　under　the　surfacecould　wreak　widespread　devastation。　much；　too；　depends　on　the　nature　of　the　subsoil；　thequake’s　duration；　the　frequency　and　severity　of　aftershocks；　and　the　physical　setting　of　theaffected　area。　all　this　means　that　the　most　fearsome　quakes　are　not　necessarily　the　mostforceful；　though　force　obviously　counts　for　a　lot。

the　largest　earthquake　since　the　scale’s　invention　was　（depending　on　which　source　youcredit）　either　one　centered　on　prince　william　sound　in　alaska　in　march　1964；　whichmeasured　9。2　on　the　richter　scale；　or　one　in　the　pacific　ocean　off　the　coast　of　chile　in　1960；which　was　initially　logged　at　8。6　magnitude　but　later　revised　upward　by　some　authorities（including　the　united　states　geological　survey）　to　a　truly　grand…scale　9。5。　as　you　will　gatherfrom　this；　measuring　earthquakes　is　not　always　an　exact　science；　particularly　wheninterpreting　readings　f